The present invention relates to hydro-transformers whereby one drive member or several drive members are supplied with fluid from means for outputting hydraulic energy.
From the prior art as represented in xe2x80x9cHydrostatische Antriebe mit Sekundxc3xa4rregelung, Der Hydraulik Trainer Band 6xe2x80x9d, Vogel-Buchverlag Wxc3xcrzburg, 1989, hydro-transformers are known to present a fixed displacement unit hydraulically connected with the drive member and a variable displacement unit hydraulically connected with the impressed-pressure system. The fixed displacement unit is a means having the function of fixed displacement pump/fixed displacement motor, whereas the variable displacement unit is a secondary-control means having the function of variable displacement pump/variable displacement motor. The shafts of the fixed displacement unit and of the variable displacement unit are mechanically coupled with each other.
If a cylinder with a unilaterally acting load is used as a drive member, use of a hydro-transformer has the purpose of realizing predetermined cylinder velocities upon extension and upon retraction of the cylinder independently of cylinder load, and the occurrence of lower losses.
In operation of a like conventional hydro-transformer, for extension of the cylinder the rotational speeds of the connected shafts of variable displacement unit and fixed displacement unit are therefore predetermined, and a rotational speed control is performed through adaptation of the stroke volume of the variable displacement unit. The fixed displacement unit herein operates as a pump, and the variable displacement unit as a motor. Upon retraction of the cylinder, on the other hand, the variable displacement unit operates as a pump and the fixed displacement unit operates as a motor, with rotational speed control concurrently also taking place. Moreover the variable displacement unit effects a recovery of energy which is fed into the impressed-pressure system. The rotational speed control may be carried out both hydraulically, such as in patent specification U.S. Pat. No. 4,819,429, or electronically during extension and retraction of the cylinder.
Conventional hydro-transformers have the drawback of necessitating high expenditure in terms of apparatus technology for their application, for which reason they are primarily employed in the upper power range such as, e.g., in large-size excavators.
The drawbacks of the prior art are to be overcome by the present invention.
The present invention thus has the object of furnishing a hydro-transformer whereby predetermined volume flows are achieved at a drive member independently of the load on this drive member, energy recovery from the drive member is possible, expenditure in terms of apparatus technology is lower than in conventional hydro-transformers, and a favorable efficiency is attained.
This object is achieved through a hydro-transformer in accordance with claim 1.
What is provided is a hydro-transformer including any type of hydraulic machine, one port of which may optionally be connected hydraulically with a pressure line through a work port of the switching valve and which preferably is a gear motor. The other port of the hydraulic machine is hydraulically connected with a hydraulic line leading to a drive member. The switching valve is controlled through a control means in accordance with a measured characteristic value which characterizes the volume flow of the hydraulic machine. Herein the work port is optionally connected with the pressure port or with a port at which a lower pressure prevails than in the hydraulic machine. In a case where the drive member is a cylinder, the latter may thus be extended in accordance with the volume flow. When the cylinder is retracted, on the other hand, fluid may be conveyed into the pressure line in accordance with the volume flow, resulting in energy recovery. On account of the low expenditure in terms of apparatus technology, the range of application of hydro-transformers may be expanded substantially.
It is advantageous if driving of the switching valve is performed in such a way that the characteristic value characterizing the volume flow is substantially constant. Thus a cylinder may be extended and retracted at a constant velocity independent of a load.
As a characteristic value for the volume flow it is possible to utilize the rotational speed of the hydraulic machine. Thus a measurement value must be determined which may be tapped simply and cost-effectively, whereby the cost for the hydro-transformer is influenced favorably.
It is furthermore advantageous to optionally connect the one port of the hydraulic machine via the switching valve, in a first switching position, with the pressure line or, in a second switching position, with a drain line, for the relatively high volume flow into the drain line results in short switching periods of the hydro-transformer according to the invention.
Although with the aid of the present invention it is possible to also effect extension of a cylinder as a drive member exclusively, it is favorable to use a hydraulic machine having two volume flow directions and two directions of rotation, for in this way energy transmitted to the fluid during retraction of the cylinder may be partly recovered.
If, e.g., a pressure present in the pressure line is twice as high as the one at the drive member, the switching valve should be actuated in such a way that the switching valve will remain in the first position for the same duration of time as in the second position.
Moreover between the other port of the hydraulic machine and the line leading to the drive member, another switching valve may be provided which is capable of optionally switching the hydraulic connection between hydraulic machine and drive member. Hereby it is possible to supply fluid through a pressure line having a lower pressure to a drive member having a higher pressure.
Preferably the further switching valve is switchable in such a way that the drive member will be in hydraulic connection either with the hydraulic machine or with the drain line. In order to avoid cavitation, it is sensible to bias the pressure in the drain line. For the same reason an anti-cavitation valve may be provided. The large difference between the pressure in the drain line and the one in the hydraulic machine brings about high responsiveness of the hydro-transformer in accordance with the invention.
A high clock frequency at a low volume flow in the pressure line reduces pulsation at the drive member, while the switching loss is kept low in the pressure line by means of a low clock frequency at high volume flow. Clock frequencies above or below a predetermined value should therefore be selected depending on an increase above or a drop below a predetermined volume flow.
The attenuation behavior of the hydraulic machine may be improved by rotation symmetrically attaching an additional mass to its shaft. Owing to the higher moment of inertia, a low clock frequency at a high volume flow is supported. In this way the switching losses are reduced.
The hydro-transformer according to the invention is preferably employed in mobile hydraulics. Thus a cost-efficient hydro-transformer is now also available in mobile hydraulics, thus rendering control of a drive member nearly independent of the presence of an accurately determined pressure level in the pressure line. As a result, lightweight and low-cost hydrostatic accumulators may be employed in mobile hydraulics to a larger extent.
Further developments of the present invention are the subject matters of the remaining subclaims.